MM0704, Lesson 2
Figure 2-1. L-Section, Low-Pass Filter.
perform the required function such as supplying a picture to a TV screen, causing music in a radio, or positioning an
antenna or missile.
In the low-pass filter shown in figure 2-1, note that the output voltage is developed across the capacitor. Recall the
formula
Note that XC is large for low frequencies and small for high frequencies. Voltage is directly proportional to
impedance, and there will be a large voltage developed on the capacitor at low frequencies and a small voltage
developed at high frequencies.
This filter can be designed to develop almost all of the input voltage at low frequencies. The filter is then passing the
low frequencies because the output voltage at low frequencies is large enough to perform the desired function. Very
little voltage at low frequencies will be lost on the coil because it has very little opposition to low frequencies.
T-section. The operation of the T-section (figure 2-2) is similar to that of the L-section. However, the addition of L2
in the circuit, with its corre-
Figure 2-2. T-Section, Low-Pass Filter
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